Andrei Purgin, deputy prime minister of the self-proclaimed Donetsk People's Republic, said that "the Ukrainians themselves have bombed the road constantly with airplanes and Grads."

He added, "We don't have the ability to send Grads into that territory."

Ukrainian military spokesmen said the bus convoy had been in an area of fierce fighting between government forces and the separatists.

The U.S. State Department strongly condemned the attack. But spokeswoman Marie Harf told a news briefing that the United States could not confirm who was responsible.

Meanwhile, Russian Foreign Minister Sergei Lavrov says agreements had been reached with Ukraine and the International Red Cross that clear the way for Russian humanitarian shipments into eastern Ukraine.

Lavrov made the remarks in Berlin on August 18 -- a day after he met for five hours with the foreign ministers of Ukraine, Germany, and France.

A large convoy of trucks that Russia says is filled with humanitarian aid for residents of Luhansk and Donetsk remained in Russia near the Ukrainian border early on August 18.

NATO and Kyiv have expressed concerns Russia would try to deploy military forces in Ukraine under the guise of humanitarian deliveries.

The International Red Cross has agreed to deliver the aid if there are no military escorts, but it was waiting for pro-Russian separatists in eastern Ukraine to guarantee the safety of Red Cross workers.

Meanwhile, Lavrov said that he "cannot report positive results" at the Berlin talks on establishing a cease-fire between Ukrainian government forces and pro-Russian separatists in eastern Ukraine.

German Foreign Minister Frank-Walter Steinmeier said the talks in Berlin on August 17 had been "difficult," but said he thought and hoped there had been "some progress on certain points."

He said the ministers would report back to leaders in their capitals and possibly agree on August 18 or August 19 about how to continue talks.

Kyiv has accused Russia of trying to send fresh military equipment across the border into eastern Ukraine during the weekend.

Ukrainian military spokesman Lysenko said "a convoy of military equipment, including three Grad rocket launchers, had crossed into the territory of Ukraine" and continued on to the village of Nizhniy Nagolchyk in the Luhansk region.